Selective carrier off noise suppression circuit



Aug. 16, 1949. M BROWN 2,479,305

SELECTIVE CARRIER OFF NOISE SUPPRESSION CIRCUIT Filed April 25, 1945 Fig.2. H 29 1 I FILT DISCRIHINATDR 8 5 55 7 {HEB-78G, l

27 9 IF. 7 32 26 Y as I8 14 I9 u I J 25 59 7 I6 :2 5T T "+4o L I.- Luann; M T2 Fiig fi Inventor:

Geofge M. Brown His Attorney.

Patented Aug. 16, 1949 SELECTIVE CARRIER OFF NOISE SUPPRESSION CIRCUIT George M. Brown,

Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application April 25, 1945, Serial No. 590,226 1 Claim. (01. 2506) My invention relates to noise suppression circuits for radio receivers and it has for its primary object to provide a new and improved radio frequency circuit in which a reproducer connected therewith is inoperative when no signals are being received and is connected for the reproduction of a desired signal only when a keying or selecting signal is likewise received.

It is customary in receivers for use as emergency communication equipment and in which a message is received at irregular intervals to include in'the receiver circuit a noise suppression, or squelch, circuit which operates to silence the reproducer of the receiver in the absence of a received carrier, thus to prevent the reproduction of noises present in the receiver circuits. It has been found, however, that such noise suppression or squelch circuits, when operated by an incoming carrier, are likewise responsiveat times to spurious and undesired signals such as those produced by diathermy apparatus which, in operation, radiates a signal of suitable frequency and other characteristics. It is an object of the present invention to provide a new and improved noise suppression circuit which silences a reproducer of a radio receiver in the absence of a carrier and which is responsive, not to the pres ence of a carrier alone, but also to a particular modulation frequency to permit reproduction of desired received signals.

It is another object of my invention to provide a new and improved noise suppression circuit for a radio receiver which is responsive both to a particular modulation frequency of a received carrier to permit reproduction of signals and to noise greater than a predetermined intensity to prevent reproduction of signals.

It is still another object of my invention to provide a new and improved selective communication system.

One of the features of my invention is the use, in conjunction with a noise suppression or squelch circuit in a radio receiver, of a frequency respon sive circuit responsive to a particular modulation frequency to render inoperative the noise suppression circuit and to permit reproduction of desired signals.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in which Fig.

1 diagrammatically represents a portion of the circuit of a radio receiver embodying my invention; Fig. 2 represents a modification of the circuit of Fig. 1 which is effective to silence a receiver in the absence of a carrier having particular modulation characteristics, as well as in the presence of undesired noises; and Fig. 3 is a diagrammatic illustration of a selective communication system embodying the features of the invention. I

In the portion ofthe radio receiver circuit shown in Fig. 1, a high frequency carrier wave received at an antenna I is amplified in a radio frequency amplifier 2 and mixed in" the usual converter 3 with oscillations from a local oscillator 4 to produce signals of intermediate frequency which are amplified in an intermediate frequency amplifier 5. The signals at the output of the amplifier -5 are demodulated in a demodulator 6 and supplied to the control grid 1 of an audio amplifier 8. Signals at the output of the amplifier 8 may be further amplified in audio amplifiers 9 and may be supplied to a signaling device, such as a loudspeaker [0. The elements of the receiver thus far described may be conventional in form and are indicated by the appropriately numbered rectangles and other symbols in Fig. 1. The re-" ceiver, moreover, may be adapted for the reception' of either amplitude modulated carrierwaves or'wave length modulated waves, such as frequency modulated or phase modulated carrier Waves. i

The selective carrier off noise suppression circuit of my invention comprises a band pass filter which may be, for example, a parallel tuned circuit consisting of an inductance I l and a capacitor l2 connected between ground and the output of detector 6 through a decoupling resistance [3. Signals of the frequency of the timed circuit ll, l2 are rectified in the diode portion of a diodetriod-e vacuum tube l4, the signals being impressed between the diode anode l5 and the cathode IS. The rectified signals of the frequency of the tuned circuit H, I! are developed across an output resistor l1 and supplied through a filter comprising resistance l8 and bypass capacitor l8, which removes the A. C. component, to the control grid N1 of the triode section of the tube [4. The triode portion of the tube l4 is'rendered normally conductive in the absence of any control signal by the adjustmentof the potential of the cathode I6 relative to the potential of the grid l9 by means of an adjustable contact 20 on a potentiometer connected across a voltage supply indicated as a battery 2|, the potentiometer comprising serially connected resistances 22, 23, 24. The small amount of current passed by the triode section of device M in the absence of'a control signal produces a potential drop across the anode resistance 25, which potential is supplied through a resistance 26 to the control grid 1 of audio amplifier 8 to bias this amplifier to cutoff and to prevent the reproduction in the reproducer I!) of any noise or signal in the receiver circuits. The proper operating'potential for the anode of the triode section of tube I4 is supplied from a 3 point intermediate resistances 22, 2 3 of the potentiometer. Operating potentials for thetube 8 are provided by the potential drop across resistances 23 and 24 of the potentiometer, the

cathode of device 8 being connected to a point intermediate the resistances 23, 24; the anode being connected to the positive terminal of the battery 2|, and the control. grid 1.. being connected to a point intermediate the resistances 22, 23.

The described carrier off noisesuppressioncib cuit is selective or frequency responsive in character. That is, in the absence of a received carrier wave modulated by. the frequency to which the tuned circuit. I I, 12 is resonant, the normally conductive triodehportion ofthe tube I4, which opratesas a, switching tube .or D. Q. amplifier produces a potential drop across resistan ce fi which biases the audio amplifier;8 to cutoff preventing the reproduction'ot anyundesired signals or noise in the output circuits of the re; ceiver. In the presencegof a received carrier modulated by the controlling vtone of the resonance frequency oithe tuned circuit H, I2, a potentials is developed across resistance through the action of the diode portionof the tube 4; to bias thetriode. portion-of this tube to cutoff. No potential is then developed across resistance,.,2Suandthe audio amplifier 8 is operativel-to reproduce the audio signals appearing atthe output oi the demodulator 6,; The circuit described, the fore s c r of noise prpression device which is sensitive, not to;the presence of a c rr nthe rece chii r th r t h presence of. the a ier plus. es lelfregu n y o the modulation oi thecarrien; This single frequencywmay ,beeny e d a re en y r be a sub. or sun rs ic requ en re er bl t i a relatively low;frequency which permits filtering out from the output circuits of the receiver by a simp h shrees t r; 1' c n One o the a va ta of h srp it u v ype of e t e e r e eeii nei uppres i nweir st t t prevents. c en 'nsle he 'no ez p re sion circuit by either random noise or by a ny undesired signals of the freguency of the carrler and, which ma .v ad t d 1 i 7, te ri ea b e transmitter which is associated with a different communication system, but wihch is operating on the same or nearly thel same carrier frequency. The circuit likewise isinsensitive to spurious signals, such as those produced by diathermy apparatus or otherstypes 9f electrical equipment. i In the modificatig of my receiver circuit shown in Fig. 2, the noise suppression circuitthere illust ree s re ee'eei e netten to ipe ie l edulation frequency tosperrnit reproduction of sigpals inthe output circuits, but-likewise is responsive to undesired noise signals having characteristics difielrent from those of desired signals which may be present when boththe carrier and selecting or keyingsignal are being received. In Eig. v 2 elements corresponding to elements of the circuit of Fig. 1, given corresponding referencevnunierals The receiver circuit therein depicted is of the. type adapted, for the reception of a carrier wave, which is modulated in pha s e,;or frequency, and it, comprises A av limiter zg having an inputcircuit 2 9 coupled to the output circuit 30 of the intermediate frequency amplifier he high frequency carrier waves, after being s1 il5 je cted; to amplitude limitation. in the li ite 2 e e d m d ate n e d ct qrtdiscrimii'iator s1 eilqtne same frequency si nals are reproduced across an output resistance 32 The resistance 32 may be a yolume control potentiometer having angadjustabl arm 33 for supplying audio signals to the control grid 1 of the output or audio amplifier 8. The demodulation signals are likewise supplied through the decoupling resistance 13 to the tuned circuit ll, 52; Potentials developed across the tuned circuit are impressed across the electrodes of the diode portion of the vacuum tube M to provide, as in the circuit of Fig, 1, a negative potential which biases the control grid 19 of the triode portion of the tube l4 to cutoif, reducing the potential drop across resistance 25 and permitting amplification in the audio amplifier 8 of the signals s p iedet t fi f sli ,hl ne -T' e mea s r ile n th uiiiii r s.... o th ec ive eiiF e 2. her eee ieee e ire c e ee si nal hevin file iea .e i ierie ies ei ierin i ec c ese 9 th 1 nals are similarto those, described and claimed in my Patent 2 ,2 6l,fi 3, assigned to the assignee of the present er l eei m an i e e i d ,,eie eet r,.e. r si Malihiih a tuned i ut eir i 3i eeum dt ihfe W995 22 i: put circuit tiil of ths ntens m r The c test vine. l c ,l r v. circuit 3 appears acrqss a diodeload resistancg 35, the potentials across the resistance b 'g supplied to the grid p; a tridd e contr, l fier 38. The anode pur es; of varnpl 38 flo s through the resistance 25 and is effective to 'ro; duce a negative potentialat the grid 1 to bi audio amplifier 3,,to Y triode amplifier- 1 s o er h e m nswear the w ease: of the circuit 34 1'sprovided wan s adjustable pos t esbn n n efyei-e s enes on a resistance Min. the potentiometer connected across the batteryfi. i I c i In the absence. of a rec vd c er, a .po si tive bias is supplied to trolgrid 3 -1 of {the m l fi r 38 th id i fiz i e-ir is e c si eildfi connected I between the grid 3 17 and ithe ;Var,iab1e contact 45 so that during siiichp'eriods t s 'cir cuit is likewiseeffectiue to biajs' theta udl o airipli her a to utoir t prevent; eproqiietidii 013.1111- desirable noises in th'eputput [of the receiver. When a carrier of the desird frequncy, is received a unidirectional potential-due tothe rec; tification of the ea ring aeveieped acr s "as resistance 3% and the diode. rectiiierpircuitto oppose the positiv'e threshold was sppued io the control'grid 3i frointhe tap dll. Bkadjustment of the position of tap 40 when a carrier cidesire dr strength is received, the trio'de 38 is rap: dered non-conductive so, that no urr nt iron; this triode ilowsthrough fies 1 F3 5? to present translation, of signals'ib y the amp ifier 8. If carrier is frequency modulated ith. a s: ected tone of the frequency to which he re nenq c113 cuit H, i2 is tuned, the current flow through as amplitude modulation of this carrier wave. This amplitude modulation noise is rectified by the diode of the diode detector circuit 34 and reproduced across the diode load resistance 36. The by-pass capacitance 43 connected across load resistance 36 is suificiently small that this composite circuit has a relatively short time constant so that negative peaks of the undesired amplitude modulations are rectified and supplied to the control grid 31 of the amplifier 38 to cause pulses of anode current to flow through the resistance 25 and bias the audio amplifier tube to cutoff. Preferably, the capacitor 44 connected across the resistance '25 is sufliciently large that the composite circuit has a relatively long time constant. As a result, the pulses of current flowing through resistor 25 maintain a negative voltage across capacitor 44 which maintains device 8 non-conducting as explained more fully in my above-mentioned patent. In this fashion, undesired noises are not reproduced across the resistance 25 to operate the audio amplifier 38 in accordance with their positive peaks. Instead, the pulses of anode current in device 38 caused by undesired noise amplitude modulations of a received carrier wave maintain a charge on capacitor 44 which biases device 8 t cut-off.

The magnitude of an incoming carrier wave which is permitted to bias the control amplifier 38 to cutoff is determined by adjustment of contact 40 on biasing resistor 4|. Adjustment of this contact establishes a threshold level for this portion of the squelch circuit. Similarly, the intensity of the audio control signal of the frequency of the tuned circuit H, 12, which is required to operate the selective portion of the noise suppression circuit, is determined by adjustment of the contact 20 on resistance 22. a

From the foregoing description, it is apparent that the circuit of Fig. 2 requires the fulfillment of two conditions before the output circuits of the receiver will open to translate audio signals, namely, the presence of a control tone of the resonance frequency of the tuned circuit ll, I2 to establish a voltage across this circuit, and the absence of undesired amplitude modulations on the incomin phase or frequency modulated carrier wave.

The selectivity feature of my improved carrier off noise suppression circuit permits the construction of an improved communication system in which various receivers may be responsive to dif ferent control frequencies and a transmitter is supplied with a means for selecting the modulation control frequency transmitted. Such a system is shown schematically in Fig. 3 in which the transmitter 45 having an output antenna 46 is provided with a plurality of oscillator circuits illustrated as the three circuits 41, 48, 49, each tuned to a different frequency F1, F2, F3 and which may be selectively connected to modulate the output carrier wave of the transmitter by means of a key 50. The system may include a corresponding number of receivers l53, each including, respectively, a corresponding tuned circuit 54-56 tuned to the same frequency as a selected one of the plurality of oscillator circuits 41-49. This system provides muting of the receivers 51-53 at all times when the key 50 is not connected to the one of the tuned circuits 41-49 to whose frequency the corresponding circuit 54-56 of the receiver is tuned. To this end each of the receivers 51-53 includes the selective carrier-ofl noise suppressor circuit of Fig. 1. In this way, each of the receivers til-53 is sensitive or responsive to its own control frequency only. The keying arrangement of the transmitter 50 may likewise include a contact position 51 in which all control tones are transmitted at one time so that communication between the transmitter 45 and all of the receivers at the same time is permitted. Such a system is particularly desirable for use in emergency communication equipment, such as in police receivers, where it is desirable to establish communication either with a particular one of the receivers or with all of the receivers at the same time.

In the foregoing description, the tuned circuit II, 12 is shown as a simple tuned circuit. However, any other type of filter circuit of the sharp band pass type may be substituted, if desired.

While I have shown particular embodiments of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may be made and I aim in the appended claim to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

A receiver for carrier waves modulated by desired signals and a keying signal comprising means for demodulating said waves to reproduce signals therefrom, means for amplifying said demodulated signals comprising a first electron discharge device having a control electrode, a resistance connected to said control electrode, a pair of electron discharge devices having their output circuits serially connected with said resistance and, the output currents of said devices producing across said resistance a biasing voltage for said electrode to render said amplifier inoperative, a tuned circuit connected to the output of said dem'odulating means, said circuit being tuned to the frequency of said keying signal, means responsive to voltages developed across said tuned circuit to reduce the current flowing in one of said pair of devices, and means responsive to a received carrier wave to reduce the current flowing in the other of said pair of devices, whereby said biasin voltage is reduced and said first device is rendered operative to amplify said signals.

GEORGE M. BROWN.

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